The present invention refers to microstructure technology and especially to electromechanical components.
Electromechanical components are components which electrically detect or electrically cause a mechanical effect. Examples of electromechanical components are sensors for linear accelerations, rotary speed sensors, force sensors, pressure sensors and also microvalves or micropumps.
Acceleration sensors, for example, i.e. sensors for detecting a linear acceleration, or rotary speed sensors for detecting an angular acceleration, normally include a movable mass which is connected to a fixed frame through at least one spring beam. When an acceleration sensor is subjected to an acceleration, the spring beam will deform elastically and the mass will be deflected. This deflection can then be detected making use of a large number of known methods, such a capacitive, inductive, optical etc. methods.
Microvalves, however, normally have a movable, elastic structure which, in response to the application of a suitable electric signal, will reduce or enlarge the size of a flow path for a fluid, i.e. which will cause as a mechanical effect a limitation of the amount of fluid flowing through.
Micropumps are, however, normally provided with a diaphragm which is elastic or elastically suspended so as to change a volume. A micropump will normally also be provided with valves so as to achieve via said change in volume a conveyance of a defined amount of fluid. It follows that the mechanical effect in the case of micropumps is transport and dosage of a fluid.
Pressure sensors or force sensors may also be provided with an elastically deformable diaphragm, which is elastically deformed, i.e. xe2x80x9cdeflectedxe2x80x9d, to a certain degree in response to a specific pressure; just as in the case of the acceleration sensor, this deflection can be detected in various ways so as to obtain an electric signal indicative of the pressure applied. All the above-mentioned electromechanical components comprise an active part, which is elastically deformed by the outer mechanical effect or the elastic deformation of which leads to the mechanical effect.
Such electromechanical components can comprise an integrated means for converting the mechanical effect into an electric effect or for converting an electric effect into a mechanical effect. Only by way of example, the known electrode structure, e.g. in the form of fingers or in the form of a diaphragm, should here be mentioned; this electrode structure comprises a first group of electrodes connected to a movable part, and a second group of electrodes connected to a fixed part relative to which the movable part moves. The two groups of electrodes are arranged in an interleaving mode of arrangement in such a way that a deflection of the movable part relative to the fixed part results in a change in the distances between the electrodes, said change leading to a change in the capacitance of the electrode arrangement. This change in capacitance is e.g. a function of the acceleration acting on the movable part. In the case of a pressure sensor, the mechanical effect can be caused e.g. by a change in the distance between two planar electrodes in the sense of a plate capacitor. This change in capacitance can be measured making use of an alternating voltage.
Electromechanical components of this type are normally produced from silicon material in miniaturized form making use of the silicon-based technology which proved to be efficient in wafer processing. Silicon-based technology permits mass production which resulted in a wide range of use of e.g. capacitive acceleration sensors which have been produced using silicon-based technology; such acceleration sensors are in particular used in the field of automotive engineering, where acceleration sensors for airbag systems should especially be mentioned.
In the case of such silicon sensors, the inertial mass is suspended from thin springs and provided with electrode structures defining together with fixed similar electrode structures a capacitor whose capacitance changes in the case of acceleration, whereby the acceleration can be detected electronically. Silicon acceleration sensors are produced e.g. in polysilicon surface mechanics by the firm of Bosch in Reutlingen. In the case of this technology a wafer with sensor chips is produced and subsequently connected, e.g. by means of the anodic bonding method, to a cover wafer which has been prefabricated in a suitable manner again by means of silicon-based micromechanical techniques, so that the sensitive micromechanically patterned silicon sensor structures will be protected. Subsequently, the composite wafer with the encapsulated sensor chips is diced. The individual sensor chips are then installed together with an electronic chip in a suitable housing making use of standard methods in the field of microelectronical technology so as to obtain the finished sensor system. The sensor systems can then be further processed like purely electronic components.
Advantages of these silicon acceleration sensors are the small physical size of the sensor and, consequently, of the chip, the fact that they can be produced in batch production processes as well as the high long-term stability and the accuracy in view of the advantageous properties of the silicon material used.
One disadvantage of such systems is the fact that, due to the very small dimensions of their sensor structures, when e.g. electrode structures are intended to be used as groups of fingers, and in view of the so-called sticking effect, it is necessary to protect such sensors against particles and moisture by a virtually hermetic seal. Another disadvantage is that, in spite of batch production and the build-up technique used in the field of electronics technology, the manufacturing process in its entirety is still very expensive, since, in addition to the electronic chip, also two silicon wafers must be produced, connected and diced by micromechanical methods.
Although silicon-based technology has gained great acceptance, which resulted in more moderate prices for the whole clean room systems and which has already led to a high degree of automation, it should still be pointed out that a complete clean room as well as adequately trained staff are necessary for wafer processing. It follows that a decisive cost factor is not the material itself, but the production outlay, which is essentially determined by the systems required and the labour costs incurred.
DE 44 02 119 A1 discloses a micro-diaphragm pump, the diaphragm being produced from titanium and the valves from polyimide. Alternatively, the diaphragm may consist of polyimide having a heating coil applied thereto .
U.S. Pat. No. 5,836,750 discloses an electrostatically driven mesopump comprising a plurality of unit cells. A pump diaphragm can be produced from metal-coated polymers, from metal or from a conductive flexible elastic polymer.
DE 197 20 482 A1 discloses a micro-diaphragm pump having a diaphragm which consists of PC or PFA. A piezo-actor can be provided on a brass sheet which is, in turn, applied to the pump diaphragm.
It is the object of the present invention to provide less expensive electromechanical components and methods for producing the same, which still have mechanical and electrical properties comparable to those of silicon components.
In accordance with a first aspect of the present invention, this object is achieved by an electromechanical component comprising: a polymeric body including a mechanically active part and a frame; and a metal layer which covers the mechanically active part at least partially so as to mechanically stabilize the same, said mechanically active part including a spring beamconnecting the frame to a mass which moves when said spring beam bends; and said metal layer encompassing the spring beam substantially completely, with the exception of the locations where said spring beam is connected to the frame and the mass, so as to mechanically reinforce said spring beam
In accordance with a second aspect of the present invention, this object is achieved by a method for producing an electromechanical component comprising the steps of: forming a polymeric body including a mechanically active part and a frame, said mechanically active part including a spring beam connecting the frame to a mass which moves when said spring beam bends; and forming a metal layer covering the mechanically active part at least partially so as to mechanically stabilize the same, in such a way that the metal layer encompasses the spring beam substantially completely, with the exception of the locations where said spring beam is connected to the frame and the mass, so as to mechanically reinforce said spring beam.
The present invention is based on the finding that for producing electromechanical components at a really moderate price, it will be necessary to take leave of the established silicon-based technology. In accordance with the present invention, a polymer material is used as a starting material; making use of e.g. injection-moulding and/or embossing (stamping) technique(s), which has/have gained widespread acceptance as well, this polymer material can be processed such that almost arbitrary shapes and structures are obtained. In addition, polymer materials are normally very moderate in price. The decisive advantage, however, resides in the manufacturing technique. The machinery required for processing polymers is much less complicated and, consequently, much less expensive than the respective machinery used in the field of silicon-based technology. Depending on the respective composition, also polymer materials have elastic properties which can be used for producing spring beams having defined deflection properties.
Polymer materials are, however, problematic insofar as plastic materials of this kind have flow properties leading to serious problems with regard to the long-term stability, unless precautionary measures are taken. According to the present invention, this problem is solved in that mechanically active parts of the polymeric body of the electromechanical component are provided with a metal layer. A plastic/metal composite system is produced in this way, which can achieve properties that are almost as good as those of a component consisting completely of metal or of silicon. This is due to the fact that the outer metal surfaces have a stronger influence on the mechanical parameters, such as the stiffness and the areal moment of inertia, than the plastic core. For the metal layer itself, gold can be used by way of example. For reducing the costs still further, a metal layer consisting of nickel, copper etc. may, however, be used as well. The mechanically active parts of the acceleration sensor described are the spring beams through which the seismic mass is suspended from the fixed frame. In the case of electromechanical components having diaphragms, the mechanically active part also includes the diaphragm which is elastically deformable and which, due to the flow properties of the plastic material, would have an insufficient long-term stability if it were not provided with a metal layer.
In accordance with a particularly preferred embodiment, the electromechanical component consists of a two-component polymeric body comprising a first part consisting of a first polymer material which is adapted to be metallized in a wet-chemical process, and a second part consisting of a second polymer material which is not adapted to be metallized in a wet-chemical process. The necessary metallizations can be defined in this way by a double-shot injection moulding process, i.e. the metallization of the mechanically active parts which serves to improve the mechanical stability of these parts and also the metallizations which are necessary for converting the mechanical effect into an electric signal, such as finger structures, capacitor plates, and also the necessary conducting tracks of the electromechanical component leading to an internal electronic circuit, which is inserted in or secured in position on the polymeric body in a hybrid way, or to an outer plug.
The essential advantage of the method according to the present invention is an extreme reduction of costs in comparison with electromechanical components which have been produced making use of silicon-based technology.
The minimum structural sizes which can nowadays be achieved by processing plastic materials are, at least at present, still substantially larger than the sizes that can be achieved by silicon-based micromechanics. This will impair primarily the dimensions of the springs and the distances between capacitor electrodes. In order to minimize the electric noise of the sensor system, a minimum capacitance must be obtained; in the case of silicon-based technology this must be achieved through very small distances between the electrodes. In accordance with the method according to the present invention, however, this need not be purchased at the cost of an ever increasing miniaturization and the problems entailed thereby, but it can be obtained by increasing the physical sizes, since materials which are much less expensive than silicon are used and since the preferred injection moulding process does not involve any substantial limitations of the height of e.g. oscillating masses, whereas the use of polysilicon definitely leads to such limitations.
On the other hand, the moulding/machining technique using polymer materials has, as is generally known, also the potential for a production of structures which may also have sizes in the micrometer range. For this purpose, the injection moulding process is preferably combined with an injection/embossing process or with the known hot-embossing process.
The larger physical shape and physical size of the electromechanical component according to the present invention entails the advantage of a reduced sensitivity to particles and contamination. In addition, the whole metallized surface may be covered with a dense, thin gold layer so as to increase the robustness and so as to make the sensor system less sensitive to humidity and environmental influences so that the demands which have to be satisfied by an encapsulation will be much lower than those in the case of silicon components.
The method used for forming the metal layers is preferably the method of chemical metallization without making use of external current. This method may, advantageously, be combined with the method of reinforcing the metal layers by electroplating; by controlling the metal thickness in the case of a reinforcement by electroplating, the electrode distance for the electrode structures as well as the natural frequency of the sensor element can be controlled precisely and optimized for the relevant field of use. By controlling the amount of metal that is grown, the method according to the present invention also provides the possibility of determining very precisely the mass of the movable, inertial structure in the case of an acceleration sensor or the mass and also the modulus of elasticity of a diaphragm in the case of microvalves and micropumps, respectively. In addition, it is possible to increase the seismic mass in the case of inertial sensors by encompassing an inserted metal body with polymer material by injection moulding.
Finally, the whole range of possibilities of injection moulding technology is provided, e.g. the use of alignment pins/holes and of snap connections for establishing non-releasable connections or the use of sealing edges which, by means of injection moulding, are formed integrally with the component to be sealed and/or the use of externally inserted rubber gaskets; these possibilities are extremely economy-priced in comparison with silicon-based technology and almost the same effect can be achieved by them.
Last but not least, the production process comprises a small number of steps in comparison with silicon-based technology, whereby the reject rate in the production process and, consequently, also the costs can be kept low.